The present invention relates to an air-fuel ratio control system for emission control system of an internal combustion engine mounted on vehicles, and more particularly to a system for an internal combustion engine provided with a two-barrel carburetor.
The emission control system for purifying exhaust gases by a catalitic convertor with a three-way catalyst is provided with an O.sub.2 -sensor for detecting the oxygen concentration of the exhaust gases, a feedback control electronic circuit for judging the output of the O.sub.2 -sensor and for producing a driving output and an electromagnetic valve operated by the driving output for controlling amount of air to be supplied to the carburetor thereby to control air-fuel ratio of the air-fuel mixture to the stoichiometric air-fuel ratio.
The two-barrel carburetor comprises a primary side and a secondary side which is operated in heavy load conditions for increasing the output power of the engine. The secondary side also must operate to control the air-fuel ratio in heavy load conditions. Therefore, electromagnetic valve must be provided for each side. In the control system in which two electromagnetic valves are provided for one side four electromagnetic valves must be provided.
However, the electromagnetic valve is required to act at a high frequency, for example, 40 HZ and to have a high reliable operation and durability. As a result, the electromagnetic valve to meet such requirements is expensive and, in addition, the control system for a plurality of electromagnetic valves is complicated in construction and operation.
Considering now operation ranges of both sides of the two-barrel carburetor, when the engine is operated in the operating range of the primary side, the air-fuel ratio control for the main metering system and slow speed system of the secondary side is not necessary. On the other hand, when the engine is operated in the operating range of the secondary side, the control for the slow speed system in the primary side is not necessary. In addition, the range of the slow speed system in the secondary side is very narrow. Therefore, the air-fuel ratio control for the slow speed system is not always necessary, in view of the fact that fuel is supplied through the main metering system of the primary side.